SD1000IS or SD870IS or SD850IS?

I have a Canon SD630. I like it, but I'm thinking about upgrading to something that is going to give me better image quality and some image stabilization.

I'd like to stick with the Canon SD series, because I like the form-factor and ease of use. Plus, I already have an extra battery and camera case. These seem to be the most commonly recommended models:

SD1100IS
SD780IS
SD850IS

Price is not a really factor, as they are all within my range. Really, my only concern is image quality.
- Which one do you all think will give me the best general, overall image quality?
- And are any of these going to be significantly different than the SD630 in that regard?
- And am I overlooking an SD model that I should be considering?

I've owned the SD850 and it takes great Canon images. I shot side by side with my S5 and I couldn't tell which was from which camera. Very impressive for a tiny camera. The only issue I found was as the light gets low, the camera goes up to f5+ fast. But that's typical of all compact cameras. Got to love the S5's ability to open to f3.5 all the way up to 400mm!

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